1. Field of the Invention
This invention relates to improved endoscopes and endoscopic methods employing phase conjugate imaging techniques.
2. Discussion of the Prior Art
Endoscopic or least invasive surgery has many advantages over conventional "open" surgery. Patients who have undergone endoscopic surgery rather than open surgery experience vastly less trauma and much faster recovery, leading to improvement in quality and reduction in cost of health care. These advantages have spurred extensive development of endoscopes.
The term "endoscope" as used herein refers to an elongated optical probe capable of presenting a visible image of the interior of a body cavity, joint, organ or the like to a surgeon by way of an eyepiece or on a video screen. The endoscope is typically introduced into the body cavity through a bore in another device also typically referred to in the art as an endoscope (or as an endoscope sheath) including a light source as well other bores for introducing surgical instruments, water, air, suction and the like. Endoscopes as optimized for various surgical procedures are referred to as arthroscopes, colonoscopes, bronchoscopes, hysteroscopes, cystoscopes, sigmoidoscopes, laparoscopes and ureterscopes.
Endoscopes typically consist of a distal objective for forming an optical image of the interior of the body cavity, bone, joint or organ, a transfer module (sometimes termed a "relay section") for transmitting the image from the distal end of the probe to its proximal end, and an ocular at the proximal end of the transfer module for presenting the image to an eyepiece, a video camera or the like. Typically the ocular contains the movable focusing components of the endoscope.
The art has for some years sought to develop a suitable disposable endoscope. The surgical requirement of absolute sterility is difficult to satisfy with conventional endoscopes as these complex instruments are not readily amenable to conventional sterilization techniques. The spread of infectious disease is of particular concern and requires that care and caution be employed during the sterilization process. Accordingly there is a strong need for a suitable disposable endoscope, that is, one made sufficiently inexpensively as to be cost effective for disposal after single-patient use.
One of the difficult tasks in designing a satisfactory endoscope is that of designing the transfer module. The transfer module must be capable of transmitting the image formed by the objective to the ocular without significant loss of brightness or sharpness. Early designs included numerous glass refractive elements, each requiring extensive polishing and costly anti-reflection coatings. The high cost of manufacture precludes use of these designs for disposable endoscopes.
Accordingly, it is an object of the invention to provide an endoscope which can be manufactured inexpensively so as to be cost-effective for disposability after single-patient use, while not suffering optical performance losses when compared to conventional endoscopes employing complex designs too costly for disposable, single-patient use.
The angular width of the field of view of an endoscope is equivalent to the solid angle from which light rays are gathered by the objective. Typically it is desired that the field of view be centered about a viewing axis forming an angle to the axis of the elongation of the probe. In this way a greater effective field of view is provided; that is, by rotating the probe about its axis of elongation, the surgeon can scan over a larger effective field of view within a body cavity or the like. In order that the axis of the field of view forms an angle with the axis of the probe, a prism may be disposed at the distal tip of the endoscope. Such prisms typically comprise several internally reflecting surfaces to direct light rays received along the axis of the field of view along the axis of the probe. The manufacture of such prisms has in general involved exacting assembly of several costly glass elements, rendering probes incorporating prisms relatively complex and expensive. It will be apparent that an endoscope with a wide field of view is more useful than one with a narrow field of view. Accordingly, it is an object of the invention to provide an endoscope having a relatively wide field of view without requiring a prism at the distal tip of the endoscope probe and eliminating any necessity of rotating the endoscope.
As examples of prior art endoscope probes illustrating one or more of the deficiencies of the prior art mentioned above, reference may be made to the following patents.
U.S. Pat. No. 3,257,902 to Hopkins shows an optical system for an endoscope employing a number of cylindrical rod-like glass lenses in the transfer module of the probe. This design has the deficiency that the rod-like lenses are costly to form, as many individual glass surfaces must be separately polished.
U.S. Pat. No. 4,025,155 to Imai shows an improvement on the Hopkins transfer module employing field and relay lenses. The Imai transfer module is also relatively complicated and difficult to construct.
U.S. Pat. No. 4,138,192 to Yamasita shows a forward viewing optical system for an endoscope including a prism as generally discussed above. The Yamasita prism is relatively complex and expensive to manufacture.
U.S. Pat. No. 4,165,917 to Yamasita et al shows objective assemblies for endoscopes which are relatively complex and costly to manufacture.
U.S. Pat. No. 4,168,882 to Hopkins shows an improvement on the original Hopkins transfer module design of U.S. Pat. No. 3,257,902 referred to above. The improved Hopkins design is also unduly complex and expensive.
U.S. Pat. No. 4,195,904 to Yamasita shows a complicated prism structure for providing a retrograde viewing system for endoscopes.
U.S. Pat. No. 4,755,029 to Okabe shows an objective lens for an endoscope including an element formed of a gradient refractive index (GRIN) material. This design reduces the number of elements in the endoscope at the expense of increasing their complexity of manufacture by use of the GRIN material.
Finally, U.S. Pat. No. 4,964,710 to Leiner shows a transfer module for an endoscope using plano-ended glass rods and molded plastic lenses intermediate the glass rods.
More recently, there has been filed commonly-assigned U.S. patent application Ser. No. 07/833,416 in the name of Broome for a disposable endoscope. The disclosure in that patent application is incorporated herein by reference and relates to an endoscope design wherein substantially all elements of the elongated probe having curved surfaces are molded of plastic such that substantially all the glass elements are plano-ended. This design substantially simplifies manufacture of the endoscope and is cost-effective for single-patient disposable use. The Broome design further features a molded plastic prism for increasing the effective field of view of the endoscope without involving a costly multiple-element glass prism. The disposable probe of the Broome endoscope is designed to be used in conjunction with a non-disposable focusing ocular comprising several glass elements. Despite the substantial improvement provided by the Broome design, there remains as always a desire for further simplification and reduction in cost of the endoscope.
The present invention seeks to further simplify the endoscope design disclosed in the aforesaid patent application and other prior art endoscope designs by utilizing phase conjugate imaging techniques. In essence, a phase conjugate optical filter is a device capable of receiving a number of rays at random angles of incidence and redirecting those rays along paths essentially inverse to the paths of the incident rays. A reflective phase conjugate filter reflects the incident rays precisely back along their incident paths, while a transmissive phase conjugate filter retransmits the incoming rays along ray paths making angles of transmission, with respect to a plane of symmetry of the phase conjugate transmissive element, equal to the angles of incidence at which the corresponding incoming rays meet the plane of symmetry.
An example will assist in understanding the operation of a phase conjugate optical filter.
Reflection takes place at an ordinary plane mirror such that the angle of reflection of the exit ray is precisely equal to the angle of incidence of the incident ray. Thus, if one looks in a plane mirror, one's eye detects rays of light from objects having been incident on the mirror at precisely the angle from which the rays were reflected by the mirror. Accordingly, one sees one's own eye in a plane mirror only when directly looking at the mirror; that is, only then is the incident ray at precisely 90.degree. to the surface of the mirror, so that the angles of incidence and reflection are both 90.degree. . Objects off the perpendicular are seen when a ray from the object is incident on the mirror at precisely the same angle as the ray reflected from the mirror meeting one's eye. For this reason it is possible to see objects imaged in a mirror; that is, because there is a precise one-to-one correspondence between the rays incident on the mirror and the rays received by the mirror from one's eye, an image can be formed. When such a one-to-one relation does not exist, an image cannot be formed. For example, when rays from an object are received by the eye from a variety of directions, a diffuse image is formed, such as from frosted glass or a similar diffusive surface.
By comparison, a phase conjugate reflector has the property of reflecting an incident light ray received from substantially any incident angle back precisely along the incident ray path. A bicycle reflector is a simple example of a phase conjugate reflector. Light incident on the bicycle reflector from any direction is reflected back toward the source. Thus, if one is driving a car at night with the headlights illuminated, hence providing a directional beam, one can see the light reflected from a bicycle reflector even though the headlight beam is not perpendicular to the surface of the reflector. It will be intuitively apparent that if the bicycle reflector were replaced by a plane mirror, one would only see reflection of light from one's headlights under very limited circumstances, that is, when the light beam from the headlights happened to be incident on the mirror substantially perpendicular to its surface, so that the reflected light would return essentially along the path of the incident beam.
A bicycle reflector exhibits the phase conjugate property by provision of multiple-faceted reflector structures, wherein three reflecting planes meet at perpendicular angles to one another, forming "internal corners". Such internal corners have the phase conjugate property, i.e., a light ray incident at any angle on an internal corner formed of three reflectors meeting one another at right angles will reflect back along the direction of the incident ray. The same principle is used in radar reflectors commonly mounted on wires or like structures of small cross-section to ensure that a radar receiver "sees" the structure, and in other applications.
The phase conjugate property is also exhibited by certain photorefractive solids and gases under appropriate circumstances. These instances of the phase conjugate property do not involve internal reflection, but involve stimulated periodic spatial variation in the optical characteristics of the medium. For example, phase conjugation can be performed by "stimulated Brillouin scattering" and by "optical 4-wave mixing in non-linear media". See generally Yeh, "Photorefractive Phase Conjugators", Proceedings of the IEEE, vol. 80, no. 3, March 1992. This paper fully discusses the theoretical basis of phase conjugation and gives useful examples of materials which can be employed or stimulated to exhibit this property.
The properties of phase conjugators are also discussed in Shkunov et al, "Optical Phase Conjugation", Scientific American December 1985, p. 54-59. Shkunov et al provides an example of the property of phase conjugate optical elements. A coherent light beam passed through a diffusive medium such as frosted glass, if reflected from a phase conjugate reflector and passed back through the same medium, regains its original properties.
The only publication known to the present inventor specifically discussing the application of phase conjugate techniques to endoscopes is U.S. Pat. No. 4,928,695 to Goldman et al. Goldman et al disclose a system for treating diseased tissue within the body. An imaging portion of this device involves passing light distally through an endoscope along a first fiber optic. The light is reflected from the body tissue of interest to pass proximally through the endoscope along a second fiber optic, is reflected by a phase conjugate reflector, passes distally back through the second fiber optic, is reflected a second time from the body tissue, and returns proximally through the first fiber optic to be imaged on a viewing screen. The Goldman et al patent is not clear on the precise reasons for this sequence. The presence of the phase conjugate reflector in the image path between the proximal and distal traversals of the second fiber optic by the reflected light serves merely to return any image of the object to the vicinity of the object. No means is shown for forming an image of the object, or for transmitting such an image to an eyepiece or video imaging chip. Accordingly, the disclosure in Goldman et al does not teach a device capable of satisfying the aforementioned objects of the present invention.
As mentioned above, in one form of optical phase conjugation a so-called four-wave mixing technique is employed. This technique requires a coherent light source, i.e., a laser or the equivalent. It would obviously be desired to provide an endoscope not requiring such a complication. Other phase conjugate techniques employ holographic techniques also requiring a coherent light source such as a laser. For example, U.S. Pat. No. 4,921,333 to Brody et al discusses phase contrast image microscopy using optical phase conjugation. Brody employs holographic phase techniques, thus requiring a coherent light source, and relates to imaging of "transparent phase objects". It would seem that such a microscope would not be amenable to endoscopic use.
Other patents directed to the use of phase conjugation for various purposes include U.S. Pat. No. 4,500,855 to Feinberg, U.S. Pat. No. 4,750,818 to Cochran, U.S. Pat. No. 4,927,251 to Schoen, U.S. Pat. No. 4,938,596 to Gauthier et al, U.S. Pat. No. 5,018,852 to Cheng et al and U.S. Pat. No. 5,059,917 to Stevens. None of these patents relate directly to endoscopic imaging, nor appear amenable to satisfaction of the objects of the invention. Finally, U.S. Pat. No. 4,945,239 to Wist et al teaches a transilluminating system for detecting breast cancers and the like employing a phase conjugate technique, apparently to improve the image. The Wist et al device does not appear suitable for endoscopic image formation.